This has been a long criticism of the band, and speciafically Bono due to his extra curricular activities. On the one hand, you have U2 Inc, the band and the industry built round them. Ultimately, they're a huge multi million Euro/dollor organisation who has much right as any big organisation to maximise income and minimise tax legitimately. Hundreds of corganisations use the same tax minimisation methods U2 Inc use.
On the same note, Bono and the boys are rich beyond most people here's means, which they have worked long and hard for, like them or hate them, and have as much right to spend and overspend on whatever takes their pleasure, from expensive guitars (well they are the tools of their trade I guess) to expensive boats, houses, holidays. I'm earning significantly less than the guys but still would be peeved if someone told me I couldn't spend my money on a holiday because I give to charity.
Where the crux of the debate stands is Bono's worthy cause of world debt and his calling on Governments to use whatever methods to reduce such. I see this as a separate issue to their personal wealth or U2 Inc's business strategy, but they don't quite reconsile with Bono's activism. If U2 Inc or Bono Inc (as opposed to their personal wealth) were to give some demonstration in order to support his beliefs, we would tend to believe in his causes more effectively.
Let us not forget that the richest people in Ireland pay little or no tax - the poorer you are here, the more tax you pay. Rich people advising to drop debt is an ironic term here and credibility is advising such things has to be supported by hard and clear action other than the occasional rant or "wave your mobile/cell in the air"